I. Technical Field
The present invention relates generally to brain stimulation systems and methods. More specifically, the present invention relates to systems and methods that may enhance memory consolidation by stimulating neurophysiological events during wakefulness and during sleep.
II. Background
Electroencephalography (EEG) records the neural activity of electrical potential across cell membranes, which are detected through the cerebral cortex and recorded by a plurality of electrodes. The changes in electrical potential in the cortex contain rhythmical activity, which typically occur at frequencies of about 0.5 to 70 cycles per second (hertz). While awake, fast, random signals are predominantly generated at low voltage and mixed frequency. While asleep, more predictable signals are generated at a low voltage and predictable frequencies over predictable periods.
Five distinct brain wave patterns that are commonly detected during an EEG recording are delta waves (e.g., about 0.5-3 hertz), theta waves (e.g., about 3-8 hertz), alpha waves (e.g., about 8-12 hertz), beta waves (e.g., about 13-38 hertz), and gamma waves (e.g., about 38-70 hertz). Many of these frequencies may be observed in a subject's sleep cycle. A sleep cycle may be defined as a progression of brainwave patterns that may be seen while a subject is sleeping. Generally, subjects undergo several sleep cycles per night, each lasting around ninety minutes. Each progression of brainwave patterns during the sleep cycle may be referred to as a stage of the sleep cycle. Generally, each sleep cycle progresses consecutively through stage I sleep, stage II sleep, stage III sleep, stage IV sleep (stage III sleep and stage IV sleep may be grouped together and referred to as slow wave sleep), briefly back to stage II sleep, and then rapid eye movement (REM) sleep.
During stage I sleep, a subject's brain waves slow in frequency transitioning from alpha waves to theta waves. During stage II sleep, a subject's brain waves slow further and include various bursts of activity such as sleep spindles and K-complexes. Sleep spindles, as seen on an EEG recording, are brain wave patterns that begin low in amplitude and gradually increase amplitude before gradually decreasing over a second or two. Sleep spindles may also be referred to as a crescendo-decrescendo pattern. In general, sleep spindles have a frequency of about 12-14 hertz. K-complexes are brain wave patterns that include large, relatively-slow waves (e.g., 1-2 hertz) and may occur during stage II sleep. During stage III sleep, a subject's brain waves slow further in frequency and may be defined by a period in which delta waves are less than 50 percent of the total wave patterns. During stage IV sleep, a subject's brain waves slow further still and may be defined by period in which delta waves make up between 20 and 50 percent of the wave patterns. During REM sleep, a subject's brain waves increase in frequency to the gamma frequency similar to the brain waves observed during waking consciousness.
Further, during REM sleep, various bursts of sawtooth waves may be observed. The sawtooth waves that may be seen during REM sleep may precede a burst of rapid eye movements. Sawtooth waves, as seen on an EEG recording, look like a series of shark fins that oscillate at the theta frequency. Although REM sleep is characterized by actual rapid eye movement, periods of little to no eye movements may occur during REM sleep (tonic REM), which are then punctuated by bursts of rapid eye movement (phasic REM).
Waking consciousness is generally experienced neurophysiologically at a brainwave frequency of about forty hertz. The amygdala is part of the limbic system that judges emotional relevance of an experience. When the amygdala and/or the rest of the limbic system experience an event that has enough emotional relevance, the event is temporarily stored in the hippocampus. A subject's brain hippocampal wave frequency is generally about 3-8 hertz (the theta frequency) when such events are temporarily stored in the hippocampus.
Electrooculography (EOG) records the ocular activity of the electrical potential from the retina, which consists of an electrically-charged nerve membrane. EOG signals can be measured by placing electrodes near an eye. Motion of an eye may cause a measurable change of electrical potential between two or more surface electrodes.
Electromyography (EMG) records the muscular activity of electrical potential across muscular membranes, which range between about 50 microvolts to about 300 millivolts (depending on the muscle under observation). Typical repetition rate of muscle unit firing is about 7 hertz to about 200 hertz, depending on the size of the muscle, the type of muscle, etc. EMG signals may be recorded within a muscle (i.e., intramuscular EMG) or on the surface a subject's skin outside of a muscle.
A subject's EOG and/or EMG may also be useful in determining the sleep cycle of a subject. For example, when phasic burst of EOG eye movements are seen during low EMG activity along with simultaneous low voltage, mixed frequency EEG activity, the subject is likely to be in REM sleep.
Physical tasks (e.g., trampolining), learning tasks (e.g., learning a foreign language or learning Morse code), and visual tasks (e.g., visual field inversion or visual discrimination tasks) have been shown to demonstrate increases in REM sleep following successful learning. Brain wave recordings in animals have shown that the same brain areas that are activated during learning while awake are again activated during that night's REM sleep. For example, the brain wave frequency recorded in the hippocampus of rats while learning “wheel running” is in the theta frequency. The same brainwave pattern and frequency, i.e., the theta frequency, may be seen the hippocampus of the same rats during subsequent slow wave sleep and then during subsequent REM sleep. Further, this correlation has also been witnessed in rats performing other learning behavior, such as running wheels and mazes, and in other animals, such as zebra finches as they are learning and rehearsing songs.
Transcranial electric stimulation (TES) may deliver electrical current stimulation to the brain. When TES is used through the scalp to stimulate the frontal lobes during slow wave sleep over thirty minute periods, memory word pairs learning during wakefulness may be improved. It was concluded that the effects of transcranial direct cortical stimulation enhanced the generation of slow waves and thus facilitated the processes of neuronal plasticity.
Performance of a simple finger-tapping task was, in one study, improved by about twenty percent if the subjects were allowed a night of sleep between training and retesting. Further, high correlation existed between post-sleep performance and the amount of stage II sleep obtained in the last quarter of the sleep night.
Subjects tested on a series of procedural motor tasks, e.g., a pursuit rotor task, a motor task involving dexterity with a ball and cup, a direct trace task, and the fine manual dexterity game “Operation,” have been shown to have an increase in the total number of stage II sleep spindles. Subjects that were not exposed to the tasks showed no change in the number of sleep spindles.
Clicking noises delivered to subjects at the same time as the subjects undergo the sleep spindles of stage II sleep and as the subjects ascend from slow wave sleep towards REM sleep have been shown to increase central nervous system excitability.
Transcranial magnetic stimulation (TMS) refers to a noninvasive excitation of neurons in the brain by utilizing magnetic fields to induce electric currents the brain. An example of TMS may involve placing a treatment coil that generates a magnetic field near a subject's head. The magnetic field may induce an electrical current in the brain causing neurons to fire, which may induce various chemical changes in the brain.